An experiment was carried out with sugar cane at the Goat and Rabbit
Research Centre, Hatay, Vietnam, from January 1993 to December 1995 to study the effect of
row width (75, 100 or 150 cm), plant material (stem cuttings or tops), and removing or
leaving the dead leaves, on biomass yield and juice quality (°Brix).

Yield of cane stalks was higher when stem cuttings, rather than tops,
were used as planting material. The effect was most pronounced in the first harvest year
and became less with succeeding harvests. The cost of seed was also reduced by 9-10%
compared with the traditional way used by the farmers in North Vietnam. Increasing
population density by reducing inter-row spacing to 75 cm led to higher yields of biomass
with no reduction in?Brix or extraction rate of the juice. Mulching with the dead leaves
increased yields, the difference being more marked in the second than in the first year
and even more apparent in the third year (6.3, 20 and 30 % increases for mulching in
plant, 1st and 2nd ratoons, respectively). Mulching also improved soil fertility and
increased the amount of carbon sequestered in the soil, indicating that mulching is a
positive alternative to the traditional way of removing the dead leaves. Soil fertility
increased steadily with increasing ratoons indicating that the growing of sugar cane does
not exploit soil nutrients but, in contrast, has beneficial effects on growth of
subsequent crops.

In recent years a considerable amount of research has been directed at
increasing biomass quantity and quality of agricultural crops used as animal feed. Sugar
cane is a perennial crop which can be used for animal feed as well as sugar production
(Preston and Murguetio 1992). It has a high leaf area index and a high photosynthetic
efficiency under strong sunshine, more than any other crop in the tropics (Bassham 1978).
The individual and combined effects of certain management practices-- planting date, row
spacing, planting depth, fertilizer rate, pest control and irrigation -- have a great
impact on the growth and yield of sugar cane. The number of plants at harvest time is a
major determinant of biomass yield, and a density of 50-70,000 plants/ha was recommended
by Tran Van Soi (1988 ). This was a general recommendation to account for factors such as
variety, branching ability , climate and soil conditions. If the plants are too close,
there may be too many shoots which will reduce the efficiency of the parent plants and
class 1 branches while too large a space between rows will lead to a waste of the area and
of solar energy (Duong Duc Thang 1991). In the hilly land of Vietnam, higher production
was obtained with the same plant density but arranged in narrower rows (Nguyen Huy Uoc
1987) compared to the traditional method of planting. Other researchers also reported
higher yields from narrow spacing (90 and 60 cm) compared with wide spacing (130, 140 and
180 cm) (Sharma 1982; Irvine et al 1984; Singh and Singh 1984; Gonzalaz et al
1989; Arvind Misra etal 1990).

Soil organic matter is an important feature of soil fertility. Ways of
increasing soil organic matter are by growing a cover crop (green manure) or by mulching
with compost or crop residues. These practices also help to control weed growth. Mulching
has been shown to increase the yield of tomatoes and tuber plants (Kaniszewski 1994;
Pakyurek et al 1994, Khalak and Kumaraswamy 1993 ; Shin et al 1993). The
method gave higher yields in fruit trees, vegetables and crops such as mango, sweet
pepper, bananas, guava and maize ( Farre et al 1993; Siwek etal
1994; Singh and Singh 1992; Sarad-Gurung et al 1994; Sandhu et al 1992).
Mulching by crop residues was superior to polyethylene sheets (used to control weeds) in
terms of incremental cost-benefit ratios ( Khalak and Kumaraswamy 1993). There are several
reports of improvement of soil fertility as a result of mulching with crop residues
(Sandhu et al 1992; Kitou and Yoshida 1994; Arzeno 1992).

The hypotheses to be tested in the present study were:

Reducing row distance would lead to increased yield of biomass

Planting with stem cuttings rather than the growing point would increase the longevity
and the yield of sugar cane

Returning the dead leaves to the soil (mulching) would increase biomass yield and soil
fertility

Materials and methods

Location

The experiment was carried out from January 1993 to December 1995 at
the Goat and Rabbit Research Centre in Bavi district in a hilly area some 60 km North-West
of Hanoi with an average slope of 3-8o. The soil composition at the
experimental site was as follows: pH 5.3, K2O 0.06%, P205
0.09%, N 0.14%. In general, the soil is of low fertility and the organic matter has been
reduced to very low levels by erosion. Drought occurs frequently in the area.

Mulching (returning the dead leaves to the soil surface) or no mulching (taking away the
dead leaves)

Harvesting at 10 months or 12 months after planting

Row spacing was the main plot and plant material and mulching were the
sub-plots. The area for each experimental plot was 90, 80 and 75 m² for row spacings of
150, 100 and 75 cm, respectively. The size of the recorded area was 60 m² for all
treatments. The total number of plots was 36, giving a total experimental area of 3,569
m² (2,940 m² for recording data). The plant material was set in double continuous rows
with 8, 10 and 12 tonnes/ha for 150, 100 and 75 cm spacing between rows, respectively.

Cultural practices

Fertilizer was applied according to the normal practice by farmers in
the area. The amounts of cattle manure and chemical fertilizers applied are shown in Table
1.

Table
1: Applications of cattle manure and chemical fertilizers

Year

Manure

N

P2O5

K2O

(tonnes/ha)

------(kg/ha)----------

1993

20

200

100

200

1994

15

150

80

150

1995

10

100

60

100

The dressings of nitrogen were based on government recommendation of N
200 kg, P2O5 100 kg and K2O 200 kg/ha for 100 tonnes cane
stalk. It was also based on results from Kanwar etal (1989), Ravindra et
al (1989) and Chapman et al (1992) where regimes of 150-225 kg N/ ha gave
highest cane and commercial sugar yields.

The sugar cane variety used was POJ 3016 (originally from Java,
Indonesia). There were sporadic infections with insects which facilitated invasion of the
leaf blade by black fungi. An insecticide "Wafatox" at 1 g/litre concentration
was applied locally to control the insects. Weeds were removed by hand at intervals but
the amounts were not recorded.

Management of the dead leaves

The dead leaves were collected and weighed every month beginning 6
months after planting. For the mulching treatment the leaves were returned to the soil
after they were weighed.

Harvesting

At harvest the experimental plots were divided into 2 parts for
harvesting. One half was harvested at 10 months after planting and the other half at 12
months. The whole plant was cut at ground level. The stalk was separated from the whole
plant by cutting immediately below the second node measured from the top. The growing
points (tops) were then separated from the leaf blades (green leaf). Each component was
weighed and sampled for chemical analysis.

Extraction of juice

Samples of the stalk from each plot (about 10 kg) were crushed by
passing them three times through a 2-roll mill driven by a buffalo. On the second and
third pass the partially pressed stalks were doubled to maximise extraction of the juice.
Extraction rate was expressed as weight of juice as a percentage of the weight of cane
stalks. The total soluble solids in the juice (°Brix) were determined using a hand
refractometer.

Biological test of soil fertility

Soil samples (from 0-20 cm depth) were taken from each experimental
plot immediately after each harvest. Equal amounts (3 kg) were put into clay pots (about 5
litre capacity) for a biological test of overall soil fertility. Three seeds of maize were
planted. After 5 weeks the maize plants were removed from the soil, washed to remove soil
from the roots and allowed to dry for 1 hour. The total fresh biomass and the maize roots
were weighed (Maria Elena Gomez 1993 personal communication).

Soil analysis

Samples of soil (from 20 cm depth) were taken from each plot after the
12 month harvest. The 18 samples corresponding to the mulching treatments were bulked and
6 sub-samples taken for analysis. The samples from the 18 non-mulching treatments were
treated in the same way. The sub-samples were analysed for pH, N, P, K and carbon by
standard methods (AOAC 1985). Estimates were made of the populations of fungi, bacteria
and actinomycetes according to "Standard methods of analysis for Soil, Plant tissue,
Water and Fertilizer" (Philippine-Los Bańos, Laguna 1980) with
asparagine-manitolagar medium for bacteria, glycerol-agar for actinomycetes and
peptone-dextrose-agar plus rose bengal and streptomycin for fungi.

Plant populations and °Brix in juice

The total numbers of stalks in each plot were counted after 3, 6 and 9
months and at harvesting at 10 and 12 months. Beginning in the sixth month (August) after
planting and subsequently at monthly intervals, drops of juice were taken from the upper,
mid and lower inter-nodes and the three samples mixed for determination of the °Brix
value.

Statistical analysis

The data were analysed by Analysis of Variance using the General Linear
Model of the statistical software by Minitab (1993). The model used was:

Yijk= µ +ai + bj + gk + (abg)ijk + eijk

Y = Yield of sugar cane

µ = Overall mean

a

= Effect of spacing

b

= Effect of plant material

g

= Effect of mulching

e = Error

abg= Interaction between spacing, plant
material and mulching

Results and discussion

Plant density

The mean numbers of plants/m² for the different treatments are shown
in Table 2. The plant densities increased as distance between rows decreased with the
effect especially notable in the first ratoon (the second year) and the second ratoon (the
third year). The number of plants was higher in the first year when stem cuttings were
used as the plant material. In the first and second ratoons there were no differences.
Mulching had little effect on plant density.

Table 2: Effect of
treatments on plant density

Mean plants/m²

1993

1994

1995

Row
spacing(cm)

75

8.2

7.4

7.1

100

7.4

6.5

4.9

150

5.6

4.3

3.0

SE

0.24

0.2

0.5

Probability

0.001

0.001

0.001

Planting
materials

Stem
cuttings

7.2

5.6

5.3

Tops

6.4

5.3

5.0

SE

0.19

0.16

0.44

Probability

0.05

0.85

0.32

Treatment
of dead leaves

No
mulching

7.2

5.0

5.3

Mulching

7.0

5.8

5.2

SE

0.19

0.16

0.44

Probability

0.48

0.57

0.58

The densities of mature plants at harvest time in year 1 varied from
56,000 to 82,000 plants /ha which is in agreement with values obtained in the traditional
sugar cane plantation (Tran Van Soi 1988), but the populations of 43,000 at 150 cm row
distance in year 2 and of 35,000 in the third year are rather low. The population of sugar
cane at 150 and 100 cm row distance decreased markedly year by year while at the 75 cm
spacing it was more stable and in year 3 it was twice that of the 150 cm treatment.

Biomass yield

Analysis of variance of the yields of the components of the edible
biomass (stalks, tops and green leaves) which can be used as animal feed (Table 3), showed
that decreasing the row spacing led to increases in yield. These results are in agreement
with reports of Sharma (1982), Irvine et al (1984), Singh and Singh (1984), Sudama
et al (1988), Arvind et al (1990), Patel et al (1990), Bharad etal
(1991) and Singh and Singh (1992).

Table
3: Effect of treatments on fresh biomass yield at the 12 month harvest in each of the
three harvest years (tonnes/ha)

Components
of yield

Cane stalks

Tops

Green leaves

Year

1993

1994

1995

1993

1994

1995

1993

1994

1995

Row
spacing, cm

75

134

103

74.4

22.4

15.8

14.9

12.9

11.1

8.8

100

120

92.5

54.3

19.4

12.4

10.4

11

8.9

5.8

150

86.4

62.6

34.8

13.7

8.2

7.7

10.5

5.5

5.2

SE

1.9

2.8

5.25

1.02

0.72

1.17

0.86

0.63

0.84

Probability

0.001

0.001

0.001

0.001

0.001

0.001

0.001

0.001

0.001

Plant
material

Stem
cutting

117

87.1

54.4

19.8

11.9

12..3

11

8.5

6.9

Tops

110

85.7

54.6

17.2

12.4

9.7

12

8.4

7.1

SE

1.5

2.28

4.28

0.84

0.6

0.96

0.7

0.51

0.68

Probability

0.05

0.68

0.97

0.037

0.56

004

0.47

0.83

0.71

Treatment of dead leaves

No
mulching

110

78.6

47.3

17

11.9

9.6

11.7

8.6

6.5

Mulching

117

94.2

61.7

19.5

12.4

12.4

11.1

8.4

6.7

SE

1.5

2.28

4.28

0.84

0.6

0.96

0.7

0.51

0.68

Probability

0.006

0.001

0.024

0.06

0.66

0.8

0.41

0.12

0.29

Planting stem cuttings, rather than tops, increased yield of stalks in
the first year, but had no significant effect in subsequent years. Mulching with the dead
leaves increased yield in all years, the difference being more marked in the second than
in the first year and even more apparent in the third year (6.3, 20 and 30 % increases in
stalk yield in the first, the second and the third year, respectively). Mendoza (1988) in
the Philippines also reported a more marked effect of mulching in the ratoon compared with
the plant crop. Work in India (Yadav et al 1994) and in Northeastern Brazil (Ball
Coelho et al 1993) proved that recycling sugar cane trash to the soil helped to
sustain yields of successive ratoon crops in sugar cane. Other favourable effects on cane
stalk yield and economic benefits from mulching with dead leaves (trash) were reported by
Mondharan et al (1990), Sathyavelu et al (1991), Sinha et al (1991)
and Mahadevaswamy et al (1994).

The reduction in yield of total biomass (Table 4) followed closely the
trends in plant population. In the third year, yield at 150 cm row distance was only half
that of the 75 cm treatment. These data, together with those in Table 3, show clearly why
farmers replant sugar cane after the first ratoon when the row width is at the traditional
150 cm.

Table
4: Effect of treatments and of year of harvest on annual fresh biomass yield
(tonnes/ha) (combined data for 3 years at the 12 month harvest)

Components

Stalks

Tops

Leaves

Total

Row
spacing, cm

75

102.

16.5

10.9

130

100

86.

13.5

8.55

109

150

59.6

9.42

7.10

76.5

SE

2.1

0.60

0.45

2.6

Probability

0.001

0.001

0.001

0.001

Plant
material

Stem
cutting

83.5

12.8

8.73

106

Tops

82.1

13.4

8.95

105

SE

12

0.4

0.33

1.5

Probability

0.28

0.82

0.69

0.39

Treatment
of dead leaves

No
mulching

76.9

12.2

8.6

98.8

Mulching

88.7

13.6

9.1

12.1

SE

1.5

0.4

0.33

1.5

Probability

0.001

0.006

0.16

0.001

Year

1993

114

18.5

11.6

144

1994

86.4

12.1

8.57

107

1995

54.5

11.0

7.03

72.6

SE

2.1

0.60

0.55

2.7

Probability

0.001

0.001

0.001

0.001

Juice extraction rate and °Brix

The monitoring of the °Brix of the sugar cane juice (Figure 1) showed
that it increased linearly from August to December. There were no consistent differences
between treatments in °Brix value or juice extraction rate (Table 5). Juice extraction
rate appeared to be higher in the 1st ratoon compared with plant and 2nd ratoon crops.

Yields of stalk were higher, and of tops and green leaves were lower
(Table 6), for harvests made at 12 months compared with 10 months. The °Brix in the juice
was lower but extraction rate was higher at the 10 month harvest than at 12 months.

Table 6: Yield and
quality of sugar cane at harvesting time (combined data of 3 years)

Harvesting time

Stalks

Tops

Green Leaves

Total

°Brix

Juice

--------tonnes/ha-------------

------%------

10 months

75.8

15.6

10.4

101.8

15.8

56.8

12 months

84.8

13.9

9.05

108.0

19.4

51.9

SE

1.4

0.43

0.28

1.69

0.16

0.47

Probability

0.001

0.004

0.001

0.015

0.001

0.001

Amounts of trash (dead leaves) and soil fertility

The amounts of dead leaves collected monthly from 6 months after
planting to the time of harvest are shown in Table 7. The highest values were recorded for
the 75 cm row spacing. There were no significant effects of plant material or mulching
practice. According to Patriquin (1982) this amount of leaf trash will support the
fixation by soil microbes of about 150 kg N/ha.

Table
7: Effect of treatment on amount of dead leaves collected between 6 months after
planting and harvest

Year

1993

1994

1995

--------tonnes/ha--------

Row
spacing, cm

75

38.1

40.1

24.4

100

38.5

34.9

21.0

150

31.8

26.7

19.5

SE

0.82

0.53

1.3

Probability

0.02

0.001

0.04

Planting
materials

Stem
cuttings

35.8

34.2

21.7

Tops

35.4

33.1

21.5

SE

0.61

0.43

1.1

Probability

0.87

0.26

0.93

Treatment
of dead leaves

No
mulching

35.3

33.1

20.6

Mulching

36.7

34.6

22.6

SE

0.61

0.40

1.09

Probability

0.34

0.27

0.22

In the biological test of soil fertility (Table 8) there were no
significant effects of row width or planting material on the growth of the maize but there
was a significant positive effect due to mulching especially by the time of the second and
third year. Maize growth in soil on all the treatments increased (P=0.001) with succeeding
ratoons indicating a positive effect of sugar cane growing on soil fertility. The effects
were more pronounced with mulching, with increases over non-mulching of 8.2, 27.8 and
37.6% in years 1, 2 and 3, respectively.

Soil fertility was also estimated on samples taken at depths from 0-20
cm (Table 9). The data show a marked improvement in soil fertility as a result of leaving
the dead leaves on the soil. There were increases in carbon, pH, N, P and K.

Table
9: Parameters of soil fertility after mulching and no mulching before planting and at
the end of 1993, 1994 and 1995

In general, most of the parameters analysed tended to demonstrate an
improvement in soil fertility, especially microbial activity. Actinomycetes and fungi have
been shown to be associated with N fixation in sugar cane (Patriquin 1982; Dobereiner
1992). Similar results were reported by Phan Gia Tan (1993) in Vietnam and Yadav et al
(1994) in India.

Other effects

It was observed that there were more fallen and broken canes at the 75
cm row distance treatment than at the wider distances. In accordance with farmer beliefs,
it appeared that the narrow distance (high plant population) made the sugar cane more
susceptible to wind damage. The thinner cane rind at the higher plant population
apparently made the cane more susceptible to wind dadmage.

Conclusions

The reported results are part of a long term study (4-5 years) on the
effect of management practices on sugar cane grown for livestock feed. The results for the
first three years show:

When sugar cane was planted by using stem cuttings there was a higher yield in the first
and the second year. The cost of seed was also reduced by 9-10% compared with the
traditional way used by the farmers in North Vietnam.

Increasing the plant density by reducing row spacings lead to higher yields of biomass
with no reduction in °Brix or extraction rate of the juice.

Leaving the dead leaves on the soil surface increased biomass production, improved soil
fertility and increased the amount of carbon sequestered in soil. Mulching was found to be
a most positive alternative to the traditional way of removing the dead leaves.

Soil fertility increased steadily with increasing ratoons. This also indicated that the
new way of growing sugar cane does not exploit soil nutrients but, in contrast, has
beneficial effects on the growth of subsequent crops.

References

Arvind Misra, Naidu K M, Gupta, M.L and Misra, A 1990

Effect of row
spacing and nitrogen level on ethanol production in sugar cane (Saccharum officinarum)
at different ages of crop harvest. Indian Journal of Agricultural Sciences, 110-114.

Arzeno J L 1992 Advantages of mulching on soil management in the
subtropical region. In Proceedings of a conference held in Villa Maria, Argentina,
146-147.

Khalak A and Kumaraswamy A S 1993 Weed biomass in relation to
irrigation and mulching, and ecolomics of mulching potato crop under conditions of acute
water scarcity. Journal of the Indian Potato Association, 185-189.

Phan Gia Tan 1993 Effect on production of sugar cane and on soil
fertility of leaving the dead leaves on the soil or removing them, National
seminar-worshop, Sustainable livestock production on local feed resouces. pp: 28-32